1. Field of the Invention
The present invention relates to a fusing device for an electrophotographic image forming apparatus, and more particularly, to a fusing device for an electrophotographic image forming apparatus having a large-sized fusing roller which supplies fusing heat to a color or high-speed laser printer.
2. Description of the Related Art
In general, an electrophotographic printer includes a fusing device which heats a sheet of paper onto which a toner image is transferred, melts the toner image in a powder state on the paper, and fuses the melted toner image on the paper. The fusing device includes a fusing roller which fuses toner on the paper; and a pressing roller which pushes the paper against the fusing roller.
FIG. 1 is a schematic cross-sectional view of a conventional fusing roller 10 using a halogen lamp 12 as a heat source, and FIG. 2 is a schematic cross-sectional view of a conventional fusing device using the fusing roller 10 of FIG. 1. Referring to FIG. 1, the fusing roller 10 includes a cylindrical roller 11 and the halogen lamp 12 installed inside the cylindrical roller 11. A TEFLON coating layer 11a is formed on a circumference of the cylindrical roller 11. The cylindrical roller 11 is heated by radiant heat generated from the halogen lamp 12.
Referring to FIG. 2, a pressing roller 13 is placed under the fusing roller 10 to be opposite to the fusing roller 10, and a sheet of paper 14 is placed between the fusing roller 10 and the pressing roller 13. The pressing roller 13 is elastically supported by a spring 13a and closely presses the paper 14 passing between the fusing roller 10 and the pressing roller 13 toward the fusing roller 10 with a predetermined pressure. In this case, a toner image 14a formed on the paper 14 in a powder state is fused on the paper 14 due to the predetermined pressure and heat while the paper 14 passes between the fusing roller 10 and the pressing roller 13.
A thermistor 15 and a thermostat 16 are installed at one side of the fusing roller 10. The thermistor 15 measures a surface temperature of the fusing roller 10, and the thermostat 16 cuts off power supplied to the halogen lamp 12 when the surface temperature of the fusing roller 10 exceeds a predetermined value. The thermistor 15 measures the surface temperature of the fusing roller 10 and transmits an electrical signal corresponding to the measured surface temperature to a controller (not shown) of a printer (not shown). The controller controls the power supplied to the halogen lamp 12 according to the measured surface temperature and maintains the surface temperature of the fusing roller 10 within a given range. When the measured surface temperature of the fusing roller 10 exceeds the predetermined value because the controller fails in controlling the surface temperature of the fusing roller 10, a contact (not shown) of the thermostat 16 becomes open to cut off the supply of power to the halogen lamp 12.
Power consumption of a conventional fusing device using a halogen lamp as a heat source is large. In particular, the conventional fusing device requires a fairly long warming-up time when the power is turned on to the fusing device. In particular, in the conventional fusing device, the fusing roller is heated by the radiant heat generated from the heat source. Thus, a heat transfer is slow, and compensation for a difference in the surface temperature of the fusing roller due to a temperature decrease caused by contacting the paper is slow, so it is difficult to maintain the fusing roller at a predetermined temperature.
Accordingly, it is difficult to apply the conventional fusing device to a printer requiring a rapid fusing heat supply, such as a color laser printer or a black-and-white laser printer for high-speed printing of 25 sheets per minute.
In addition, when the conventional fusing device having the above structure is used in the color laser printer or a high-speed laser printer, a diameter of the fusing roller increases. As a result, a new structure is required to prevent a heat loss occurring at both ends of the fusing roller, and in order to improve a heat transfer onto the paper moving at a high-speed and having an overlapped toner image, a width of a fusing nip is needed to be increased.